Adhesion Performance in the Eggs of the Philippine Leaf Insect Phyllium philippinicum (Phasmatodea: Phylliidae)

Abstract

<jats:p>Leaf insects (Phasmatodea: Phylliidae) exhibit perfect crypsis imitating leaves. Although the special appearance of the eggs of the species Phyllium philippinicum, which imitate plant seeds, has received attention in different taxonomic studies, the attachment capability of the eggs remains rather anecdotical. We herein elucidate the specialized attachment mechanism of the eggs of this species and provide the first experimental approach to systematically characterize the functional properties of their adhesion by using different microscopy techniques and attachment force measurements on substrates with differing degrees of roughness and surface chemistry, as well as repetitive attachment/detachment cycles while under the influence of water contact. We found that a combination of folded exochorionic structures (pinnae) and a film of adhesive secretion contribute to attachment, which both respond to water. Adhesion is initiated by the glue, which becomes fluid through hydration, enabling adaption to the surface profile. Hierarchically structured pinnae support the spreading of the glue and reinforcement of the film. This combination aids the egg’s surface in adapting to the surface roughness, yet the attachment strength is additionally influenced by the egg’s surface chemistry, favoring hydrophilic substrates. Repetitive detachment and water-mediated adhesion can optimize the location of the egg to ensure suitable environmental conditions for embryonic development. Furthermore, this repeatable and water-controlled adhesion mechanism can stimulate further research for biomimeticists, ecologists and conservationalists.</jats:p>

Multifunctional Adhesives on the Eggs of the Leaf Insect Phyllium philippinicum (Phasmatodea: Phylliidae): Solvent Influence and Biomimetic Implications

“…In addition to the specific body appearance, phasmids established several functional principles similar to plants—particularly to their seeds [ 2 , 3 , 20 , 21 , 22 ]. Some eggs of stick insects deploy similar mechanisms for distribution like plant seeds.…”

“…Many plants deploy polysaccharides (e.g., cellulose, pectins, or hemicelluloses) in order to either attach their seeds to a suitable place for germination [ 35 , 36 , 37 ], or to enable zoochoric dispersal of the seeds [ 23 , 24 , 25 , 26 , 38 , 39 , 40 ]. Insects in general often employ glues to attach their eggs [ 22 , 41 , 42 , 43 , 44 ]. Attaching the eggs onto their food source is a common principle in phytophagous insects [ 41 , 42 , 43 , 45 ], or ectoparasites [ 46 , 47 ].…”

“…A similar mechanism has been reported for the eggs of the Philippine leaf insect ( Phyllium philippinicum Hennemann, Conle, Gottardo and Bresseel, 2009, Figure 1 A), which is in contrast terrestrial, but dependent on its foodplants. The eggs ( Figure 1 B) of this insect are covered with an adhesive secretion, which enables an elaborate multifunctional attachment system [ 22 ]. The glue itself responds to water and first liquefies, subsequently spreads into surface corrugations, thus maximizes the actual contact area with the substrate and finally solidifies.…”

“…The glue itself responds to water and first liquefies, subsequently spreads into surface corrugations, thus maximizes the actual contact area with the substrate and finally solidifies. This process generates strong adhesion (up to 4800 times the weight of the egg itself; [ 22 ]). Furthermore, repetitive water exposure reactivates liquefaction and consequently the mobilization of the glue deactivating the adhesive ability of the glue.…”

“…Most insect glues—either on their eggs or for other purposes—are largely proteinaceous [ 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 ]. The majority of these adhesives, which were experimentally tested so far, generate strong adhesion to different natural substrates [ 22 , 41 , 42 , 43 , 44 ]. The asparagus beetle ( Crioceris asparagi (Linnaeus, 1758), Coleoptera: Chrysomelidae) attaches its eggs onto Asparagus officinalis L. (Asparagaceae).…”